Surface-Mediated Molecular Transport of a Lipophilic Fluorescent Probe in Polydisperse Oil-in-Water Emulsions.

Emulsions often act as carriers for water-insoluble solutes that are delivered to a specific target. The molecular transport of solutes in emulsions can be facilitated by surfactants and is often limited by diffusion through the continuous phase. We here investigate this transport on a molecular scale by using a lipophilic molecular rotor as a proxy for solutes. Using fluorescence lifetime microscopy we track the transport of these molecules from the continuous phase toward the dispersed phase in polydisperse oil-in-water emulsions. We show that this transport comprises two time scales, which vary significantly with droplet size and surfactant concentration, and, depending on the type of surfactant used, can be limited either by transport across the oil-water interface or by diffusion through the continuous phase. By studying the time-resolved fluorescence of the fluorophore, accompanied by molecular dynamics simulations, we demonstrate how the rate of transport observed on a macroscopic scale can be explained in terms of the local environment that the probe molecules are exposed to.

Fluorescent molecular rotor probes nanosecond viscosity changes.

Viscosity is a key property of liquids, but it is difficult to measure in short-lived, metastable samples due to the long measuring times required by conventional rheology. Here, we show how this problem can be solved by using fluorescent molecular rotors. The excited-state fluorescence decay rate of these molecules is sensitive to the viscosity of their local environment, and by combining pulsed laser excitation with time-resolved fluorescence detection, we can measure viscosities with a time resolution of a few ns. We demonstrate this by measuring in real time the viscosity change in glycerol induced by a nanosecond temperature jump. This new approach makes it possible to measure the viscosity of extremely short-lived states of matter.

Light-switchable deposits from evaporating drops containing motile microalgae.

Deposits from evaporating drops have been shown to take a variety of shapes, depending on the physicochemical properties of both solute and solvent. Classically, the evaporation of drops of colloidal suspensions leads to the so-called coffee ring effect, caused by radially outward flows. Here we investigate deposits from evaporating drops containing living motile microalgae (Chlamydomonas reinhardtii), which are capable of resisting these flows. We show that utilizing their light-sensitivity allows control of the final pattern: adjusting the wavelength and incident angle of the light source enables forcing the formation, completely suppressing and even directing the spatial structure of algal coffee rings.

A New Type of Supramolecular Fluid Based on H2 O-Alkylammonium/Phosphonium Solutions.

Here we show that by adjusting the concentration of tetrabutyl ammonium and phosphonium salts in water (≈1.5-2.0 m), hydrophobic solvation triggers the formation of a unique, highly incompressible supramolecular liquid, with a dynamic structure similar to clathrates, involving essentially all H2 O molecules of the solvent. Despite the increasing local order, the thermal diffusivity, and compressibility of these supramolecular liquids is strongly decreased with respect to bulk water due to slower relaxation dynamics. The results presented in this paper open an avenue to design a new family of supramolecular fluids, stable under atmospheric conditions, which can find important technological applications in energy storage and conversion.

Low temperature glass/crystal transition in ionic liquids determined by H-bond vs. coulombic strength.

Self-assembled ionic liquid crystals are anisotropic ionic conductors, with potential applications in areas as important as solar cells, battery electrolytes and catalysis. However, many of these applications are still limited by the lack of precise control over the variety of phases that can be formed (nematic, smectic, or semi/fully crystalline), determined by a complex pattern of different intermolecular interactions. Here we report the results of a systematic study of crystallization of several imidazolium salts in which the relative contribution of isotropic coulombic and directional H-bond interactions is carefully tuned. Our results demonstrate that the relative strength of directional H-bonds with respect to the isotropic Coulomb interaction determines the formation of a crystalline, semi-crystalline or glassy phase at low temperature. The possibility of pinpointing H-bonding directionality in ionic liquids make them model systems to study the crystallization of an ionic solid under a perturbed Coulomb potential.

Disentangling Nano- and Macroscopic Viscosities of Aqueous Polymer Solutions Using a Fluorescent Molecular Rotor.

The macroscopic viscosity of polymer solutions in general differs strongly from the viscosity at the nanometer scale, and the relation between the two can be complicated. To investigate this relation, we use a fluorescent molecular rotor that probes the local viscosity of its molecular environment. For a range of chain lengths and concentrations, the dependence of the fluorescence on the macroscopic viscosity is well described by the classical Förster-Hoffmann (FH) equation, but the value of the FH exponent depends on the polymer chain length. We show that all data can be collapsed onto a master curve by plotting the fluorescence versus polymer concentration, which we explain in terms of the characteristic mesh size of the polymer solution. Using known scaling laws for polymers then allows us to quantitatively explain the relation between the FH exponent and the polymer chain length, allowing us to link the nano- to the macroviscosity.